The present invention relates to methods and systems which use received signals from Satellite Position Systems (SPS) to locate themselves or to determine time-of-day. This invention is a continuation-in-part of U.S. patent application Ser. No. 09/074,521, filed May 7, 1998 now U.S. Pat. No. 6,052,081 by Norman Krasner, which is a continuation of U.S. patent application Ser. No. 08/794,649, filed Feb. 3, 1997, which is now U.S. Pat. No. 5,812,087 (referred to as the xe2x80x9cParent Patentxe2x80x9d). The Parent Patent is hereby incorporated herein by reference. The disclosure in patent application Ser. No. 08/842,559 filed Apr. 15, 1997 is also incorporated herein by reference.
In most situations, the methods of the Parent Patent work reliably, allowing one system (e.g. a server system) to determine a time of capture of SPS signals (such as, for example, Global Positioning System (GPS) signals) at another system (e.g. a mobile SPS receiver/client system).
In most situations of interest the time coordination method of this invention (termed xe2x80x9cpattern matchingxe2x80x9d) works reliably. In some unusual situations, there are extremely long latencies in transmitting the signal between the mobile (e.g. mobile unit 453 of FIG. 6 of the Parent Patent) and the server (e.g. basestation 463 of FIG. 6 of the Parent Patent). This can arise if the link utilizes packet communications which allow arbitrarily long routing delays. On rare occurrences such packets may arrive after a very long period of time. Such a long latency would require that the server compare the received pattern from the mobile with a very long record stored at the server. This may be computationally complex and may require a considerable amount of time to perform the necessary computations. In addition, long latencies may give rise to ambiguities associated with the repetitions in the data patterns. For example, a substantial portion of the U.S. GPS data signal repeats at 30 second intervals, and small portions may repeat at 6 second intervals. In such circumstances the pattern match procedure may produce ambiguous results.
The present invention provides methods and apparatuses for measuring time related to satellite data messages which are used with satellite position systems, such as GPS or Glonass. A method in one embodiment comprises the steps of: (1) receiving, at an entity, a first record of at least a portion of a satellite data message; (2) comparing the first record with a second record of the satellite data message, where the first record and the second record overlap at least partially in time and where the comparing is performed after determining an estimated time when the first record was received; and (3) determining a time from the comparing, where the time indicates when the first record (e.g., the source of the first record) was received at a remote entity. In one example of this embodiment, the remote entity is a mobile SPS receiver and the entity is a basestation which communicates with the mobile SPS receiver through a wireless (and perhaps also wired) link. A method of the present invention may be performed exclusively at the basestation. In an alternative embodiment, the comparison may be performed and then the estimated time when the first record was received is used to verify that the time determined from the comparing is correct.
An embodiment of the present invention for establishing receiver timing is for the receiver to form an estimate of a portion of the satellite data message and transmit this estimate to the basestation. At the basestation this estimate is compared to a record of the satellite data message received from another GPS receiver or source of GPS information. This record is assumed to be error free. This comparison then determines which portion of the basestation""s message most closely matches the data transmitted by the remote unit. Since the basestation has read the satellite data message without error it can associate each data bit of that message with an absolute time stamp, as seen by the transmitting satellite. Hence the comparison results in the basestation assigning an appropriate time to the estimated data transmitted by the remote. This time information may be transmitted back to the remote, if desired.
A variation on the above approach is to have the basestation send a clean record of the satellite data message to the remote plus the absolute time associated with the beginning of this message. In this case the remote entity compares this record to the estimate of this data which it forms by processing a GPS signal which it receives. This comparison will provide the offset in time between the two records and thereby establish an absolute time for the locally collected data.